Tension control apparatus



Aug. 20, 1968 w. c. LOVELETT TENSION CONTROL APPARATUS Filed Dec. 17, 1965 United States Patent 3,397,721 TENSION CONTROL APPARATUS William C. Lovelett, Mount Holly, N.J., assignor to CF&I Steel Corporation, a corporation of Colorado Filed Dec. 17, 1965, Ser. No. 514,546 1 Claim. (Cl. 140-147) ABSTRACT OF THE DISCLOSURE Apparatus for maintaining a minimum constant tension on a cable being tensioned by progressively larger sheaves having a constant angular velocity, particularly when the apparatus is stopped or reversed. The apparatus includes a return sheave mounted to guide the cable from the small sheaves to the large sheaves, means to support the return sheave for motion between a fixed stop position and other positions more remote from the large and small sheaves, a hydraulic cylinder connected to the return sheave support means, and a source of constant pressure hydraulic fluid for the cylinder.

This invention relates to a method and apparatus for processing wire and cables. More particularly this invention relates to a method and apparatus for processing cable to improve its tensile properties and its configuration.

Cable that has been straightened and made into an accurate helical shape is particularly suited for use in prestressed concrete construction because under the continued tensile load of such construction, there is a minimum of elongation of the cable due to its continued straightening or to a change in the position or the shape of an imperfect helix of single length wire or strand. Methods and apparatus are known for stabilizing or improving the straightness of cable by heating while it is under tension. One of the most successful methods and apparatus for accomplishing the latter comp-rises applying to a cable a controlled predetermined strain by advancing the cable around a plurality of sheaves, each rotating at the same angular velocity but having progressively increasing diameter in the direction of cable advancement. The cable is thus subjected to increasing tension as it passes through the apparatus. At the same time successive predetermined lengths of the tensioned cable are heated to a temperature which will produce a predetermined permanent elongation in the cable.

However, it has been found that if operation according to the method is stopped for any reason, the section of cable in the apparatus is improperly stabilized. To remedy such improper stabilization it is customary that the cable be reversed in direction without heating. However, when the cable is reversed, the tension on the cable decreases because of (1) the constant angularity of rotation of the sheaves and (2) the difference in sheave diameter, because the larger sheaves feed cable to the smaller sheaves when in reverse operation, and the smaller sheaves cannot take up the wire as it is fed.

Take up means for excess cable are known in the cable stressing art. However the known means make use of a return sheave which serves as a dancer to activate drag on a cooperating first pair of capstans. Such means are unsatisfactory for maintaining full operational tension on cable in constant-angularity, varying-sheave-diameter apparatus.

It is an object of the present invention to provide an improved method and apparatus for improving the tensile characteristics and the configuration of cable.

It is another object of the present invention to provide an improved method and apparatus for tensioning and straightening cables which may be started and stopped without loss of tension on the cable.

It is still another object of the invention to provide apparatus for maintaining a minimum constant tension on a cable being tensioned by progressively larger sheaves having a constant angular velocity, particularly when the apparatus is. stopped or reversed.

It is yet another object of this invention to provide an improved cable straightening method and apparatus utilizing a return sheave which, during reverse operation, serves to take up excess cable while maintaining a predetermined minimum tension on the cable.

It is a further object of this invention to provide an improved cable straightening method and apparatus for taking up excess cable during reverse operation which applies tension directly to the cable rather than to a capstan.

Other objects and further features and advantages of the invention will appear more fully from the following description. The invention will now be described in greater detail with reference to the appended drawings in which:

FIGURE 1 is a plan View, partly schematic, of apparatus illustrating the principles of the present invention; and- FIGURE 2 is an elevational view of apparatus embodying the present invention taken generally along lines 22 of FIGURE 1.

The present invention comprises the use of the return sheave under controlled conditions to maintain a predetermined minimum tension in the cable regardless of whether it is being run forward and permanently elongated, or run backward and not changing in length, or is not moving.

The objects of this invention for stabilizing cable may be accomplished by wrapping the cable around a pair of small capstan sheaves having a number of grooves, all of equal diameter, enough times toprevent any slip of the cable in relation to the capstans. From the capstans the cable is led around a return sheave and then onto a pair of large capstan sheaves having a greater diameter than the small capstan sheaves. A cable heater is disposed adjacent the cable path between the small and large sheaves. Both sets of sheaves are mounted on common shafts so that the angular rotational speed of the small sheaves and the large sheaves is the same. Additional small and large sheaves may be used if desired. The difierence in peripheral speed and the fact that the cable cannot slip causes the cable to elongate by the ratio of the large sheave diameter to the small sheave diameter. Heating of the cable aids stabilization under tension.

During normal operation the return sheave rests against a fixed stop and serves only to guide the cable. However, during the changing of reels the direction of rotation of the sheaves is reversed and the large sheaves feed cable to the small sheaves. More cable is given off by the large sheaves than can be taken up by the small sheaves. As the elongation is not elastic, an excess of cable backs up between the large and small sheaves. Under such conditions, the return sheave is moved away from the large and small sheaves to take up the excess cable and maintain the cable at a predetermined minimum tension.

Movement of the return sheave may be controlled by a hydraulic cylinder connected to the return sheave support means. Pressure in the hydraulic cylinder controls the force on the return sheave and the tension on the cable. The hydraulic cylinder may be pressurized by fluid from a hydraulic pump which maintains a body of hydraulic fluid at a predetermined pressure related to the desired tension on the cable.

After the reels are changed, the direction of cable movement is again from small to large sheaves. The force applied by the large sheaves is greater than that applied by the hydraulic cylinder. The excess cable is taken up by the large sheaves, and the return sheaves assumes its usual position adjacent the stop.

In FIGURE 1 the apparatus 10 of the present invention may be seen to be comprised of a first shaft 12 arranged for rotation in either direction as indicated by the arrow. A small capstan sheave 14 is mounted on the shaft 12 and arranged to rotate with it. A large capstan sheave 16 is also mounted on shaft 12 and arranged to rotate with it. Because sheaves 14 and 16 are on a common shaft each moves with the same angular velocity. The peripheral velocity of the sheave 16 is greater than that of the sheave 14 because the sheave 16 has a larger circumference than sheave 14. A second shaft 18 is similarly arranged for rotation in both directions and has a small capstan sheave 20 and a large capstan sheave 22 mounted on it as shown. The function and operation of shaft 18 and sheaves 20 and 22 is similar to that of shaft 12 and sheaves 14 and 16, respectively. The number of such shaft and sheave assemblies may be multiplied as necessary to produce a non slipping movement of the cable 23 which passes around the sheaves and is moved by them.

A return sheave 24 is mounted on a shaft 26 and is arranged for rotation in either direction as shown by the arrows. The shaft 26 is mounted on a piston rod 28 which is operated by a hydraulic cylinder 30. The hydraulic cylinder 30 is connected by a conduit 31 to a source of constant pressure hydraulic fluid, not shOWn, which may be a body of hydraulic fluid pressurized by a hydraulic pump to a preset constant pressure. The hydraulic cylinder 30 is mounted on any firm foundation 32, as shown. The hydraulic cylinder 30 is adapted to move the piston rod 23 back and forth in the direction of the arrow as shown. Such motion also moves the shaft 26 and the return sheave 24 back and forth in the same direction.

The cable 23 is threaded through the grooves 34 of each of the sheaves in well known fashion and passes through a heat treating apparatus 36. The forward direction of cable travel is from point A around the sheaves 14 and 20, around the sheave 24, around sheaves 22 and 16 exiting at point B.

In forward operation the cable 32 is subjected to a tensioning force between the small sheaves and the large sheaves as the wire strand moves from point A to point B. While the cable is under tension, it is heated which stabilizes the cable by causing the wires from which it is formed to straighten and conform to a helical pattern, and by imparting a reduced tendency to vary from the helical pattern when the tension is removed.

When the cable travels in the reverse direction from point B toward point A, the tensioning force on the cable is reduced. When the force due to the capstan sheaves is less than the force on the piston rod 28, the constant pressure in the hydraulic cylinder causes the piston rod 28 to move to the left as shown in the drawings and so causes the shaft 26 to move to the left. Because the hydraulic cylinder 30is under constant pressure, the piston rod 28, shaft 26 and return sheave 24 are under a constant force. The force on the return sheave 24 imparts a tensioning force in the cable 23. The relative and absolute amounts of force imparted by the capstan sheaves and by the return sheave may be varied with regard to the particular cable being stabilized.

The terms and expressions which. have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. In apparatus for improving the tensile properties and straightening cables made up of a first set of small sheaves, a second set of large sheaves, common shafts on which said small sheaves and large sheaves are mounted, means to rotate said shafts and cause said sheaves to move at a constant angular velocity, said cable being passed successively about said small and said large sheaves, and means disposed between said large and small sheaves for heating said cable, the improvement for maintaining a predetermined minimum constant tension on said cable comprising a return sheave mounted so as to guide said cable from said small sheaves to said large sheaves; means to support said return sheave so that it is adapted for motion between a fixed stop position and other positions more remote from said large and small sheaves than said fixed stop position; a hydraulic cylinder connected to said return sheave support means; and a source of constant pressure hydraulic fluid connected to said hydraulic cylinder, said constant pressure being selected so that at a particular minimum tension in said cable said return sheave is in said fixed stop position thereof and so that any reduction in tension in said cable below said particular minimum tension causes said hydraulic cylinder to move said return sheave away from said fixed stop position by an amount which will maintain the tension in said cable at said particular minimum tension.

References Cited UNITED STATES PATENTS 3,153,696 10/1964 Blanchard 72-364 1,064,936 6/1913 Scott 72 205 3,114,451 12/1963 Collins 72-183 FOREIGN PATENTS 246,846 9/ 1963 Australia.

65,488 4/ 1950 Netherlands.

CHARLES W. LANHAM, Primary Examiner.

L. A. LARSON, Assistant Examiner. 

